Among the various lead-free solder alloy choices as replacements of conventional tin-lead solders, Tin (Sn)-Silver (Ag)-Copper (Cu) alloys are currently the most popular because of their relatively good soldering performance, excellent creep resistance, and thermal fatigue reliability, as well as their compatibility with the current components. A variety of Sn—Ag—Cu solder alloys have been proposed and recommended for use by industrial organizations in different countries. For example, Sn-3.0Ag-0.5Cu (wt. %) by the Japan Electronic Industry Development Association (JEIDA) in Japan, Sn-3.8Ag-0.7Cu (wt. %) by the European Consortium BRITE-EURAM in the European Union, and Sn-3.9Ag-0.6Cu (wt. %) by the National Electronics Manufacturing Initiative (NEMI) in the United States of America. However, recent investigations on the lead-free solder alloys have shown that solder joints made from these recommended Sn—Ag—Cu alloys may be fragile and prone to premature interfacial failure under drop impact loading. Although reduction of Ag content in Sn—Ag—Cu alloys has been found to be helpful, drop test performance for these alloys is still inferior to that of eutectic tin-lead. Traditionally, solder joint reliability has been evaluated mainly by thermal fatigue performance since thermal fatigue fracture has been the critical failure mode in electronics interconnects. As the industry is pushing for device miniaturization and increased use of portable electronic products, impact reliability of solder joints in electronic packages becomes critical, in addition to conventional thermal fatigue reliability.
In view of the forgoing, it would be desirable to provide Sn—Ag—Cu based solder alloys and solder joints thereof with improved drop impact reliability.